A material is defined as being "electrochromatic" when its colour changes under the influence of an electric current passing through said material, or under the influence of an electric field. Various physical phenomena are responsable for electrochromatic effects in solids, liquids, organic or inorganic substances.
As regards the solids, most of the electrochromatic phenomena concern the formation of colour centers or the charge transfer between impurity centers, which results in the suppression of an absorption band and the creation of a new band.
As far as the liquids are concerned, the phenomena involved are generally of an electrochemical nature, which has led to designating them by the term of "electrochemichromatic phenomena". Two effects should be mainly considered:
"redox" reactions (i.e. oxidation-reduction reactions) by which non-coloured species are reduced or oxidized so as to form coloured species; PA0 a metallisation effect due to an oxidation-reduction reaction wherein the metal ions will deposit on an electrode so as to form a metallic layer. PA0 comparatively long response times, which range from a fraction of a second to about one second at ambient temperature; PA0 absence of a clearly defined threshold of the effect, since the resulting optical density is a function of the total charge having passed through the cell, and thus depends on the value of the current and the duration of its application.
The most efficient performances are exibited by systems related to the colour center effect in solids, and to electrochemical oxidation-reduction phenomena in liquids or solids.
The present invention concerns the systems based on the displacement of ions in solids or liquids.
Up to now all the known devices of this type have exhibited two particular features which limit their field of application:
The instant invention is aimed at overcoming these two drawbacks. It is based on the fact that the response time is determined by the diffusion of the ions in the material, and that the ion diffusion coefficient is a very "quick" (or steep) function of the exponential type, of temperature. According to the invention, it is proposed, in a visualization device, to heat the electrochromatic material during a short period of time, so as to decrease the duration of application of the control voltage, and thus obtain more quickly acting visualization systems. The heating may be effected by the passage of an electric current through one of the electrodes which serve to apply the control voltage to the electrochromatic material.